In applications involving silicon photonics optical inputs/output arrangements may be realized, via an adiabatic or “evanescent” coupling. The term adiabatic coupling refers to the fact that transition occurs substantially without loss of energy.
Adiabatic or “evanescent” coupling may involve one or more optical layers realized in, e.g., an optical integrated circuit (OIC) and one or more optical layers realized, e.g., in a medium/support/interposer, external to the OIC. The external medium may be conventionally attached by gluing to the OIC. Setting the distance between the optical layer(s) of the OIC and an external medium/support/interposer to a certain value facilitates making the adiabatic or evanescent coupling effective.
The working principle of such a coupling is based on asymmetric directional couplers (DC), wherein each waveguide changes its cross section and/or refractive index along a direction of propagation.
The effective refractive index neff is a number quantifying the phase delay per unit length in a waveguide relative to the phase delay in vacuum.
When the effective refractive index (or propagation constant) of a waveguide matches the effective refractive index of a further waveguide, optical coupling may take place.
Examples of adiabatic coupling may be found in:    P. Bettotti, “Hybrid Materials for Integrated Photonics”, Advances in Optics, Volume 2014 (2014), Article ID 891395, 24 pages,    S. Dhoore et al., “Novel adiabatic tapered couplers for active III-V/SOI devices fabricated through transfer printing”, (2016) OPTICS EXPRESS 24(12). p. 12976-12990,    A. La Porta et al., “Silicon Photonics Packaging for Highly Scalable Optical Interconnects”, IEEE 65th Electronic Components and Technology Conference (ECTC), pp. 1299-1304, 2015.
Adiabatic or evanescent coupling between, e.g., an optical chip and an external medium may be considered as a new approach, with few approaches for realizing such coupling being suggested in the art. Such approaches may be suitable for demo samples, however they show inherent weaknesses, possibly incompatible with reliable products.
Also, document T. Creazzo et al.: “Integrated tunable CMOS laser”, 2013, Optical Society of America, 28048-28053, may be of interest for one or more embodiments.